DE2340696A1 - EXTRACTION OF META-XYLOL BY SELECTIVE CRYSTALLIZATION - Google Patents

Description

Dr. F. Zumsteln sen. - Dr. E. Assmann Dr. R. Koenlgsberger - Dipl.-Phys. R. Holzbauer - Dr. F. Zumsteln jun.

PATENT LAWYERS

TELEPHONE: COLLECTING NO. 22 5341

TELEX 529979

TELEGRAMS: ZUMPAT CHECK ACCOUNT: MUNICH 91139

BANK ACCOUNT: BANK H. HOUSES

8 MUNICH 2,

BRÄUHAUSSTRASSE 4 / III

Case 279 854 53 / Jo

STANDARD OIL COMPANY, Chicago, 111. / USA

Extraction of meta-xylene by selective crystal! sation

The invention relates to a process for selectively separating meta-xylene from a mixture of Cg isomers, wherein particular process conditions are followed and the paraxylene is removed at an unexpectedly low rate crystallizes based on the rate at which meta-xylene crystallizes.

According to the invention, a C ₈ isomer mixture is prepared which contains:

1. An essentially eutectic mixture of meta- and para-xylene,

2. an ortho-xylene concentration adjusted to a value below the eutectic ratio of ortho and meta-xylene and

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3. a set paraxylene concentration of approximately 10 mole percent or less paraxylene in the mixture.

This mixture is then cooled to a temperature and for a time short enough to allow a crystal to develop sludge to give the amount of meta-xylene, based on the ratio of meta- and para-xylene that is usually obtained during crystallization up to thermodynamic equilibrium is obtained, is enriched, and then high-purity meta-xylene can be obtained.

The invention relates generally to a process for the separation of compounds which have low boiling point differences, and in particular it relates to a method to selectively convert metaxylene, para-xylene and related Cg aromatic compounds to separate. More particularly, the invention relates to a process for preparing purified meta-xylene with a purity of 95% and by recrystallization or partial melting, preferably with a purity of 99%, in which one Creates kinetically controlled separation conditions at which the speed or the rate or the amount of para-xylene crystallization is suppressed based on the rate or amount of meta-xylene crystallization.

In recent years there has been an increased need for feedstocks rich in meta-xylene, such as those used in the manufacture of isophthalic acid and related products. On the one hand, there are few difficulties in the separation of ethylbenzene and ortho-xylene from Cg mixtures by fractionation, mainly because of the relatively large differences in the boiling points between these C _Q constituents and related Cg constituents. On the other hand, great difficulties arise

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if you want to separate meta-xylene and para-xylene by fractionation, due to the small difference in boiling points of 0.8 ^° C. between the two components.

As a result, a variety of crystallization processes have been developed to separate meta-xylene from para-xylene. However, neither of these methods can be used to selectively engineer high concentrations of meta-xylene. A major difficulty arises in that all of these crystallization processes require that at least one Isomer remains in a metastable state. Under such conditions, of course, some formation causes one Seed crystal an immediate and extensive crystallization of the undesired isomer and one can therefore not a suitable one Reach concentration of either meta-xylene or para-xylene.

In most industrial crystallization processes for para-xylene, for example, waste filtrate streams ■ are formed, which are rich in meta-xylene (after the para-xylene has been recovered) and which contain as much as 8 to 13% para-xylene. Previous attempts to crystallize such mixtures and produce high purity meta-xylene have only had that Formation of mixtures of para and meta-xylene crystals which at best give an unsatisfactory equilibrium composition which is approximately 88% meta-xylene and contains 12 # paraxylene.

As a result, a variety of rather difficult and expensive methods have been proposed to address these difficulties dissolve and to produce high-purity meta-xylene, such as sulfonation, adductive crystallization, Clathration and Adsorption. In most cases, however, it must

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a third component can be introduced into the separation system, which in turn provides additional devices for the subsequent Requires separation and extraction of this third component. In the case of the separation of meta-xylene by selective Sulphonation requires considerable equipment just to recover and re-concentrate the sulfuric acid. Similarly, the use of HP-BF requires that for meta-xylene recovery the handling of corrosive gases and expensive special alloys for the operating devices.

Clathration processes, on the other hand, require the mechanical handling of about 8 to 10 times the amount of extraneous Material related to the components to be separated and these methods are because of energy requirements Already very expensive. Finally, adsorption processes require even adsorption processes involving an isomeric component is adsorbed on a solid adsorbent, large initial investments in adsorbents and these can are easily deactivated by impurities contained in the isomeric feedstock.

As a result of the disadvantages inherent in the known separation processes, there is a great need for a less difficult, relatively inexpensive, but very effective method to to separate meta-xylene from its isomers. Firstly, this process should make do with existing para-xylene recovery plants and secondly, it should utilize the available cooling capacity, i.e. it should take into account the fact that the Process fluids have already cooled to low temperature.

It has now been found that meta-xylene of improved purity selectively and preferably from mixtures of meta-xylene and others aromatic Cg isomers, which contain as impurities can be separated if you have special

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driving conditions, with the relative crystallization rates of meta- and para-xylene are chosen so that para-xylene crystallizes at an unexpectedly low rate of crystallization relative to the rate of crystallization of raeta-xylene. More precisely, in the process according to the invention, a mixture of meta-xylene and its aromatic Cg isomers to act as an impurity are first selected or prepared on the basis that the Mixture contains:

1. An essentially eutectic mixture of meta- and Paraxylene, i.e. meta- and paraxylene are essentially present in their eutectic ratio,

2. a set concentration of ortho-xylene in the mixture at a value below the eutectic ratio of ortho- and meta-xylene and

3. a set concentration of para-xylene in the mixture of about 10 uol% or less.

The mixture produced in this way contains adjusted or selected ones Concentrations of ortho-, meta- and para-xylene and it is then at a temperature and during a sufficient cooled for a short time to give a slurry of crystals enriched in meta-xylene, relatively to the ratio of meta- and para-xylene that is usually used in crystallization up to the thermodynamic Maintains balance.

The invention is explained in more detail with the aid of the accompanying drawings:

Fig. 1 shows a schematic flow diagram of an embodiment according to the invention, wherein an external connection

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thinning agent is added to the xylene mixtures, to adjust the concentration of paraxylene, and

2 shows a schematic flow sheet of another embodiment of the process according to the invention, the concentration of para-xylene being adjusted by a two-stage crystallization process.

According to the method of the invention, meta-xylene can be selectively from Cp isomer mixtures, the meta-xylene, para-xylene, ethylbenzene and ortho-xylene contained. Surprisingly mixtures of crystals enriched in meta-xylene can be used in the process according to the invention, be selectively and preferably crystallized from solutions containing para-xylene. Due to the unique process conditions, as they are defined in the process of the invention, it is possible to selectively paraxylene with a unexpectedly low rate of crystallization relative to the rate at which meta-xylene crystallizes. The enrichment of meta-xylene thus occurs in a kinetically regulated process in which meta- and para-xylene with relative crystallization rates, that differ, crystallize.

The selection or, if desired, preparation of a feed stream of the Cg isomers is made on the following basis. First, the mixture of C _Q isomers desirably contains meta- and para-xylene in essentially their eutectic ratio. Since the eutectic ratio of meta- and para-xylene varies as the total concentration of para and meta-xylene in the Cg mixture varies, the exact eutectic ratio used will vary somewhat. For example, in a binary mixture containing only para and meta-xylene, the eutectic ratio is approximately 87 to 13 meta-to-para-xylene.

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In the case of a co-isomer mixture that only has one common has meta- and para-xylene concentration of approximately 25%, the eutectic ratio is approximately 91 to 9 meta- to para-xylene. Because typical waste filtrate streams coming from manufacturing plants obtained from para-xylene, contain Cg isomer mixtures in which the meta- and the para-xylene essentially are present in their eutectic relationship are natural such waste filtrate streams are very desirable starting materials for the process of the invention.

After having a mixture of Cg isomers, the meta- and para-xylene contains, selected or produced essentially in their eutectic ratio, the concentration of ortho-xylene, which is contained in the mixture, to one Value below the eutectic ratio of ortho- and meta-xylene set. The concentration of ortho-xylene contained in the mixture of Cg isomers is preferred, reduced to a value below the eutectic ratio of ortho- and meta-xylene before any Crystallization or removal of para-xylene takes place. In the case of a typical waste filtrate stream, required this usually the reduction of ortho-xylene in the mixture to about 11 mole percent or less.

Finally, the concentration of the para-xylene contained in the mixture is adjusted to about 10 mol% or less, and preferably to 8 mol% or less. The lower the paraxylene concentration below 10 Hol -? *, The greater the difference between the relative crystallization rates of meta and paraxylene is of course. The effect of paraxylene concentration on the rate of paraxylene crystallization is illustrated in the following table, which shows the rate constants for the crystallization of paraxylene at different levels of concentration.

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-8th-

Table I.

Crystallization Rate Constants dc / dt = ^

Test temperature- original (PX) ^ 'k ^ ' rature ( ⁰ C) (MoIJi)

-59,212,766,4 χ 10 ~ ⁴ -67.0. 9.6 21.6 χ 10 * " ⁴

-72.4 7.2 5.1 x 10 " ⁴

^ ¹ ^ S = supersaturation = (PX) at the beginning - (PX) solubility '

(2) -Λ

^K ' in units of min

^ '(PX) = para-xylene concentration

The adjustment of the paraxylene concentration can be done by various means Procedure. A very suitable method consists in adding a diluent to the Cp mixture adds until the concentration of the paraxylene component is reduced to about 10 mol% or less. the Diluents that are most suitable are low melting solvents such as toluene, butane, or propane similar, and also easily obtained from the Cg isomers by distillation can be separated. Cg maphthenes, which are obtained as a by-product of isomerization, are also suitable diluents for the process according to the invention.

Another useful method of adjusting the concentration of paraxylene requires that the original cg-hishing a number of consecutive Is subjected to crystallizations. The first of these crystallizations - is carried out to remove para-xylene until the eutectic of meta-xylene and para-xylene is reached.

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Then the mother liquor from the first crystallization is further crystallized to form crystals with the para- / meta-xylene eutectic remove approximately 88% meta-xylene and 12 / ά para-xylene and at the same time to prepare a diluted liquor having a concentration of 10 mol% or less of paraxylene contains.

In any case, the mixture must have a Cg-set paraxylene concentration which is not greater than about 10 mole-94 ¹¹ u ^ preferably, the concentrations of less than 8 mol%. It has been found that even purer meta-xylene can be produced if the concentration of para-xylene in the Cg mixture is further reduced, for example to 7 mol% or less. For example, metaxylene with 90% purity or even greater purity can be produced by adjusting the para-xylene concentration to 8 mol%, and if the para-xylene concentration is adjusted to 7 mol%, metaxylene is obtained with a purity of 97%.

The appropriate selection of the starting mixture, i.e. the mixture, which contains meta- and para-xylene in their eutectic ratio can avoid the need to first use the meta- and adjust para-xylene concentrations to the desired Get relationship. If, for example, a waste filtrate stream of the Cg isomers is used, there is no initial crystallization required to make a Cg mixture that has meta- and para-xylene in essentially their proper form Contains ratio.

It should also be noted that the removal of ortho-xylene if necessary, may be carried out before any paraxylene crystallization in the paraxylene recovery unit takes place. For example, the ortho-xylene is preferably from the first reformate charge

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electricity to a para-xylene plant removed. is \ '! fa Abfallfiltratstrom of a para-Xylolanlage used in the inventive process, the ortho-xylene should preferably first / ground by fractionation away v before any crystallization or cooling takes place from the feedstock in the para-Xylolanlage.

After the production or the selection of a suitable Cg mixture for the process according to the invention and after setting the concentrations of ortho- and para-) §a.ol on the desired values, the mixture in a crystallizer to a temperature and during a sufficient briefly cooled to a slurry of crystals which is enriched in meta-xylene, relative to the ratio of meta- and para-xylene, which is usually in solids, which are at thermodynamic equilibrium crystallize, is present. In practical terms, this means that the treated Cg mixture for approximately 5 to 150 min and preferably cooled for 5 ms 40 min to give a slurry of crystals and liquids that is out of equilibrium and about 10 to 90 wt .-% corresponds to the amount of solid crystals that are usually obtained by crystallization up to thermodynamic equilibrium is maintained.

Surprisingly, it crystallizes during this cooling stage the para-xylene that is present in the chilled mixture, but at a rate that is sufficiently slow, based on the meta-xylene crystallization rate, and thereby the recovery of an enriched meta-xylene concentrate possible, based on the ratio of meta- and para-xylene, which is usually used in the crystallization up to to thermodynamic equilibrium.

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The selective crystallization of the meta-xylene is done by seeding with meta-xylene crystals, by reducing the residence time in the crystallizer and by maintaining small temperature differences between the Cg solution and activated by the coolant. These conditions are desirably met by using an internal coolant, such as carbon dioxide, ethane, ethylene or the like, is used, with the coolant being physically introduced into the Cg mixture will. By carefully selecting the internal coolant, the coolant can also be used as a diluent serve to bring the para-xylene concentration to the desired one Set value.

At a desired rate, the meta-Xylolkristallisation example, bis-9O ⁰ C carried out at temperatures in the range of about -65. Inoculation of the Cg solution activates the rate of crystallization of meta-xylene, although inoculation is not required to form high purity meta-xylene crystals. It has been observed that the use of between about 0.1 and 2.0 wt. 6 uniformly sized metaxylene seeds increases the rate of crystallization of metaxylene to the desired level.

The residence times in the crystallizer are similar usually regulated to use a time sufficient to form a slurry of crystals and to give liquids containing about 10 to 90 weight percent crystal solids which can be obtained under the same conditions can expect. It should be noted, however, that higher or lower temperatures may be required in order to adjust the content of the total solids and crystals to the desired values. Typically they are short Crystallizer residence times in the range from approximately 5 to 150 minutes and preferably 5 to 40 minutes at temperatures of approx.

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Fähr -65 to ⁰ C -9O necessary to obtain the desired enriched meta-Xylolkonzentrationen. For example, shorter crystallizer residence times result in lower levels of solids after cooling, but these solids contain higher concentrations of metaxylene.

The crystallization process can be carried out either batchwise or continuously. The crystallization process Devices or devices can be designed as stirred tanks or preferably as conical flow-through kettles. The recirculation the product slurry to form meta-xylene seeds is particularly desirable when using conical flow-through kettles in the crystallization process will. In the case of crystallization vessels with a stirring tank, recirculation of the product slurry is not necessary,

Finally, if a jacketed crystallization device is used to cool in the process according to the invention, instead of using internal refrigerants, the temperature difference between the Cg aromatic solution and the Walls of the crystallizer are desirably kept in the range of about 1 to 10 ° C. If you use on the other hand, an internal coolant, the desired small temperature difference between the coolant and the Cg mixture inherently achieved.

The removal of the high-purity crystals rich in meta-xylene, which are obtained in the process according to the invention from the mother liquor which remains after the recrystallization, can be made by any of a number of mechanical methods. For example, filtration or centrifugation can be used and perform similar procedures. 'Furthermore, the recovered mete-xylene crystals continue by partial melting or by another crystallization method, if desired be concentrated, with even purer meta-xylene

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obtained, while the mother liquors, which are obtained from partial melting or recrystallization, are fractionated can be used to remove diluents or can be recycled for isomerization and retreatment.

The co-mixture containing meta-xylene is prior to treatment, to remove the ortho-xylene, and before treatment, to set the par-xylene concentration accordingly for the invention Set process, a mixture of about 45 mol% meta-xylene and about 20 mol% para-xylene, about 15 mole percent ethylbenzene and about 20 mole percent ortho-xylene. This mixture can be obtained by a number of methods. Under typical operating conditions in a Paraxylene system contains a residue or waste filtrate, obtained in the crystallization and recovery of para-xylene from a Cg reformate, about 8 to 20 moles Ethyl benzene, about 8 to 13 mole percent paraxylene and about 15 to 25 mole percent ortho-xylene with the remainder being meta-xylene. Sufficient ortho-xylene is preferably removed by fractionation prior to para-xylene crystallization so that only 11 Mol% or less of ortho-xylene remains in the mixture. At this point the ortho-xylene concentration should be sufficiently small for the meta-xylene to crystallize can without first reaching the ortho-xylene eutectic.

ann the concentration of para-xylene to 8 mol? S or a lower value whatever is desired in the present invention.

In practice, the inventive method in a A variety of flowchart arrangements can be performed. Two such arrangements are shown in FIGS. For example, in the embodiment shown in FIG. 1, a typical mixture of Cg isomers, such as

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they are obtained under operating conditions in para-xylene plants, ■ (15 mol-tf ethylbenzene, 20 mol% para-xylene, 45 mol-So meta-xylene and 20 mol ^ ortho-xylene) into column 11 via line 31 and is fractionated to remove approximately half of the ortho-xylene. The overhead stream from column 11 is then passed into a first crystallizer 12 via line 32 and cooled to a temperature of about -65 ° C to recover paraxylene which is in excess of the eutectic concentration. 10 mol- ^ ortho-xylene are removed by fractionation. 90 mol / s of the Cg mixture are passed via line 32 into crystallization device 12 (or in crystallizer 12). The composition that is introduced into the crystallizer contains 16.7 moles of ethylbenzene, 22.2 UoI- ⁰ A para-xylene, 50.0 M0I-9S meta-xylene and 11.1 moles of S »ortho- Xylene. During this crystallization in the first stage, 14 mol% of para-xylene crystals are removed in the centrifuge 13 of the first stage and 76 mol of mother liquor are passed via line 33 to the crystallization of the second stage. The mother liquor has a composition of 19.3 mol% ethylbenzene, 9.7 mol% paraxylene, 58.0 mol% meta-xylene and 13.0 mol% ortho-xylene.

As indicated above, the mother liquor from this first stage of crystallization is diluted with a low-melting solvent in order to lower the paraxylene _{content of the C g mixture.} At this stage, the C _Q mixture, before cooling in the crystallizer 14, has a composition corresponding to 16.8 mol-So ethylbenzene, 6.75 mol-S-5 para-xylene, 50.6 mol-56 meta-xylene, 11, 3 mole percent ortho-xylene and 14.6 mole So diluent. This diluted mother liquor \ FIRD in the crystallizer 14 via line 34 and then cooled to a temperature of -74.7 ⁰ C for a time sufficient to provide the desired amounts of solids,

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enriched to the desired meta-xylene concentration. The meta-xylene-rich crystals are then removed via centrifuge 15 and the mother liquor is distilled in column 16 to recover the diluent. Finally, the remaining mixture of Cg isomers is recycled into the fractionation column 11 via the isomerization device 17.

The embodiment shown in FIG. 1 can also be used with other conditions, as explained in the following examples are carried out.

The following examples illustrate the invention without restricting it.

example 1

100 moles (15% EB, 20% PX, 4550 MX, 20? S OX)

Distillation, yield = 20 moles of ortho-xylene. 80 mol (18.8% EB, 25.0? $ PX, 56.2? $ MX) crystallization, -62.3 ⁰ C, yield = 14 moles of para-xylene

66 moles (22.7? $ EB, 9.155 PX, 68.2? $ MX)

Dilution, 23 mol of diluent are added at -62 ° C

89 moles (16.85 $ EB, 6.75? $ PX, 50.65 $ MX, 25.9> $ diluent)

Crystallization, -74.7 ° C, yield = 10 mol of a 95% concentrate of meta-Xjzbl

79 moles (19.055 EB, 6.96? $ PX, 45.0? $ MX, 29.25-5 diluent)
. Distillation, yield = 23 moles of diluent

56 moles (26.8% EB, 9,850 PX, $ 63.45 MX)

Total: 100 parts feed + yield of isomerized material:

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14 parts para-xylene

20 parts of ortho-xylene

meta-xylene 10 parts 95/5 meta-xylene / para-xylene-Mi snake.

From the embodiment according to the invention shown in FIG it can be seen that two stage crystallization can be used to determine the paraxylene concentration. instead of adding diluents as shown in FIG. In this embodiment becomes a mixed isomerized Cg material from the isomerizer 21 is introduced into fractionation column 22 via line 40. Excess ortho-xylene becomes taken in column 22 until the ortho-xylene concentration in the overhead flow of line 41 to 10 mole percent or less is decreased.

The overhead stream is directed to a first stage crystallizer 23 via line 41 to crystallize pare-xylene until the meta-xylene eutectic is reached. The mother liquor which leaves the centrifuge 24 via the line 42 after this crystallization of the first stage is then used as feed material for the crystallization in the second stage in the crystallization device 25. There a mixture of C _Q isomers is crystallized and the crystals have a composition that approaches the eutectic of para-xylene and meta-xylene, ie they contain, for example, 125a para-xylene and 88% meta-xylene, and these crystals v / earth removed via the centrifuge 26 via the line 43. The mother liquor which leaves the centrifuge 26 via line 44 has a set paraxylene concentration of 10 mol- ^ o or less. The mother liquor is then passed into the crystallization device 27 of the third stage via line 44, v / o it to a temperature and select

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rend a time that is sufficient to cool to the desired Amount of solids with the desired meta-xylene concentration are enriched to result. Surprisingly, the para-xylene crystallizes at a rate which is much lower than that of meta-xylene and through the centrifuge 28 are metaxylene-rich via line 45 Crystals removed. The mother liquor from this third crystallization stage leaves the centrifuge 28 via the line 46 and is then fractionated in column 29 to remove ethylbenzene and it is then either via the isomerizer 21 or recycled directly into the fractionation bag 22.

The embodiment according to the invention, which is shown in FIG. 2, can also be carried out using other conditions, as illustrated in the following two examples.

Example 2

100 pieces (15% EB, 20% PX, 45% MX, 205 $ OX)

Yield in the distillation = 17.5 parts of ortho-xylene 82.5 parts (18.2% EB, 24.2% PX, 54.6% MX, 3.0% OX) crystallization at -62.4 ° C, yield = 13.3 parts3 para-xylene

68.7 parts (21.86% EB, 8.94% PX, 65.59% MX, 3 "60% OX)

Crystallization at -68.9 ° C, yield = 24.5 parts (88.5% MX - 12.5% PX)
44.2 parts (33.95% EB, 6.98% PX, 53.46% MX, 5.59% OX) crystallization at -77.0 ° C, yield = 9.4 parts of a concentrate from 95.8 % meta-xylene

34.8 parts (43.1% EB, 7.74% PX, 42.0% MX, 7.10% OX)

Yield in the distillation = 13.2 parts of ethylbenzene

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21.6 parts (8.5% EB, 12.44% PX, 67.7% MX, 11.4% OX) voice

100 parts of feed + yield of isomerized Material:

17.5 parts of ortho-xylene
13.8 parts paraxylene
13.2 parts of ethylbenzene

9.4 parts 95.8 / 4.2 metaxylene / para-xylene mixture 24.5 parts 87.5 / 12.5 metaxylene / para-xylene mixture.

Example 3

Parts (15% EB, 20% PX, 45% M-X, 20> ö OX)

Distillation yield = 11.5 parts of ortho-xylene 88.5 parts (17.0% EB, 22.6% PX, 50.8% MX, 9.6% OX) crystallization at -65.0 ⁰ C, yield = 14 Share para-xylene

74.5 parts (20.18% EB, 8.10% PX, 60.32% MX, 11.40% OX) crystallization at -68.9 ° C, yield = 15 parts (88.4% MX - 12 , 5% PX)
59.5 parts (25.28% EB, 6.98% PX, 53.46% MX, 14.28% OX)

Crystallization at -76.7 ° C, yield = 12.2 parts of a Concentrate from 95.1% meta-xylene '47.3 parts (31.85% EB, 7.50% PX, 42.7% MX, 18.0% OX)

Distillation yield = 17.2 parts of ethylbenzene 30.1 parts (8.5% EB, 11.8% PX, 67.2% MX, 28.30% OX) total

100 parts of feed + yield of isomerized material

11.5 parts of ortho-xylene

14.0 parts paraxylene

12.2 parts 95.4 / 4.9 meta-xylene / para-xylene mixture, 17.2 parts ethylbenzene

15.0 parts 87.5 / 12.5 meta-xylene / para-xylene mixture.

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The processes illustrated in Figures 1 and 2 depend on the kinetic accumulation of meta-xylene in the solids during crystallization. Due to the unique selection of process conditions in the process of the present invention, the para-xylene crystallizes at an unexpectedly slow rate of crystallization relative to the rate of crystallization of the meta-xylene. As mentioned above, short residence times in the crystallizer in the range from approximately 5 to 150 minutes and preferably from 5 to 40 minutes at temperatures of voi
meta-xylene enrichment.

40 min at temperatures from -65 to -90 ⁰ C the desired

The residence time affects (1) the amount of solids formed in the process of the invention Relationship to the amount expected at equilibrium and (2) to the kinetic accumulation of meta-xylene in these Solids, which is explained in the following examples, where values are also given that show the effect the content of paraxylene in the feed to the crystallizer has an effect on kinetic enrichment.

Example 4

Crystallization attempts were made in a 45.4 liter (12 gallon) stainless crystallizer with a rough Viand Experimental facility carried out, the crystallizer technical Reaction vessels that are used in the crystallization of para-xylene was copied. The crystallizer was operated under the conditions listed in Table II. Samples of the filtrate were collected by filter sampling taken. The amount and composition of the solids were calculated indirectly from the difference in Analyzes between the feed material and the filtrate

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and the analyzes were carried out by gas chromatography. The solids believed to be in equilibrium Present were calculated by taking the solubility values at the actual operating temperature of meta-xylene and paraxylene used. As can be seen from Table II the crystalline solids contain more meta-xylene, than it corresponds to the composition at equilibrium. In general, the solids are enriched in meta-xylene, the shorter the dwell time. With a constant residence time, there will be greater accumulation in feed materials obtained with lower paraxylene content.

Table II

Kinetic crystallization during continuous operation with a short residence time

Charging dwell AT - ¹ Crystallization material time 'high sludge temperature ° C ( ⁰ % PX PX / MX min mung -

Coolant

⁰ C TF)

9.2 13.7 / 86.3 21 11.5 (21) -69.2 (-92.5)

9.2 13.7 / 86.3 27 14.3 (26) -69.8 (-93.8) 8.8 12.9 / 87.1 120 8.8 (16) -69.8 (-93, 8th)

7.3 12.5 / 87.5 120 8.8 (16) -75.3 (-103.8)

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Crystalline solid

Actual amount % of I-Ien- Actually % MX enrichment % of ge that liche % of men about the Loading at the same lot ge, the balance chic weight he Equiv / \ 3'0 meta-
Xylene k- is waiting weight expected mate- is tet 3.9 rials 62 83.5 6.8 10.9 67 87.5 79.6 1.5 13.6 84 85.9 80.7 2.7 20.0 81 87.4 84.4 15.6 84.7

PX = para-xylene
MX = meta-xylene

Example 5

The same crystallization in the pilot plant as described in Example 4 was carried out in a somewhat different manner in order to obtain a greater kinetic enrichment. During the continuous flow through the crystallizer, meta-xylene seed crystals were added, uir. induce nucleation. Filter probe samples were taken a few minutes after the start of nucleus formation ;? and rMw Meng «and composition of the solids mJ.rd / i ber-c -.- huet and. compared to the amount and composition that msr * he.?.m GXsickgavdeht em-marten would. The columns in Table III show that a very high concentration is obtained a few minutes after the start of nucleation.

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Table III

Kinetic crystallization after nucleation during the

continuous operation

Charging material % PX PX / MX

Time after the start of core formation in min

9.2 13, 7 / 87.3 Crystallites % of 49 6th Actually MX Anrei 9.2 13, 7 / 87.3 Actual at the 80 15th liche At the assurance 8.8 12 9 / 87.1 Amount of Crieieh 79 45 lot Same- about the 7.3 12, 5 / 87.5 % of weight 71 14th gev / icht Same 7.3 12, 5 / 87.5 Loading expect 65 21 expect weight kungsmB- ten men MX = Eieta-xylene Solids te amount hy ° ι « Crystallization terials ge tion temperature % temperature ⁰ C 83.9 7.8 ( ⁸ F) 88 _? 8th 81.1 11.5 9.2 86.8 77.3 2.2 10.9 93.9 84.6 10.6 19.7 91.8 83, -3 8.1 9.6 83.7 9.6 -69 _; 45 (-93, ¹ y) -68.3 (-90.9) -70.0 (-94.3) -73.5 (-1U0.3) -73.8 (-101.0) PX = p-xylene Example 6

A batch crystallization was carried out in a jacketed crystallizer in the laboratory, the walls of which were roughened. After 330 g of a feed material containing aromatic C _f , - "somers had been added, the crystallizer was cooled to -72.5 ° C. Several g

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7340696

of each meta-xylene and para-xylene crystals added to induce nucleation. About 30 minutes later, the slurry was placed in a centrifuge filter and 9.0 g raw solids moistened with mother liquor were recovered. The actual crystal mixture contained 97.1% meta-xylene and 2.9% para-xylene, compared to only 81.0% meta-xylene, which one would have expected in equilibrium at the same temperature in the solid. The results are in the table IV summarized:

Table IV

Kinetic crystallization in batch operation with

a small cooling gradient

PX / MX

Loading material

/ \ ° C temperature of the slurry - temperature of the coolant

Crystallization temperature

7.2 12.4 / 87.6

-72.5

Crystalline solids

Actual amount as % of the % of the amount expected for the schik equation weight

Actual amount

Expected at

balance

Enrichment over the equilibrium

/ 0

ϊ-ΙΧ

2.7

97.1

81.0

16.1

-24-

409810/1163

Claims (24)

2340B96 Patent to sayings Process for selectively and preferably separating off meta-xylene from a mixture of aromatic C ₀ isomers, characterized in that selects a mixture of meta-xylene and aromatic Cn isomers, which essentially contains a eutectic mixture of meta-xylene and para-xylene, the concentration of ortho-xylene in the mixture to a value below the eutectic ratio of ortho-xylene and meta-xylene adjusts, the concentration of para-xylene in the mixture to 10 mol% or less and the mixture cools for a time and at a temperature short enough to form a slurry of crystals and to give liquid wherein the crystals are enriched in meta-xylene relative to the ratio of meta-xylene and para-xylene, which is obtained on crystallization to thermal equilibrium. 2. The method according to claim 1, characterized in that the fraction of the para-xylene present, which crystallizes per unit of time, is significantly less than the fraction of meta-xylene present, which per Unit of time crystallizes. 3- The method according to claim 1, characterized in that the Cg aromatic isomer mixture before adjusting the concentration of para-xylene and ortho-xylene in the mixture about 15 mol-5 ° ethylbenzene, about 20 mol- ^ o ortho-xylene, -25-409810 / 1163 about 20 mol% paraxylene and about 45 mol% jo meta-xylene contains. 4. The method according to claim 3, characterized in that the concentration of para-xylene in the mixture of aromatic Cg isomers is adjusted by adding an amount of a diluent to the mixture sufficient to make the concentration of paraxylene therein to about 10 mol% or less, and preferably to less than 8 mole percent. 5. The method according to claim 4, characterized in that the diluent used is a low-melting solvent which is slightly different from the Cg isomers can be separated by distillation. 6. The method according to claim 4, characterized in that the diluent used is toluene, butane, Propane and / or Cg-naphthenes are used. 7. The method according to claim 3, characterized in that the cooling of the adjusted mixture of aromatic Cg-isomers is carried out at a temperature ranging from -65 to -90 ⁰ C. 8. The method according to claim 7, characterized in that about 0.1 to 2.0 wt .-% meta-xylene seed crystals can be added to the mixture of Cg isomers while cooling. 9. The method according to claim 7, characterized in that the residence time during which the Cg isomers be cooled, about 5 to 150 minutes. -26- A09810 / 1163 . - 26 - 73-0696 10. The method according to claim 1, characterized in that the residence time during which the Cq isomers be cooled, about 5 to 40 minutes at temperatures of about -65 to -90 ° C. 11. The method according to claim 1, characterized in that g e k e η η ze rejects that the concentration of paraxylene in the mixture of the Cg isomers to a value of about mol-? o or less and preferably to a value of less than 8 mol- $ is adjusted by adding the mixture one Subjects to series of successive crystallizations. 12. The method according to claim 1, characterized in that one is in the mixture of the Cg isomers introduces an internal coolant. 13. The method according to claim 12, characterized in that the internal coolant serves as a diluent for the mixture of the C _Q isomers to the concentration of paraxylene to a fourth of 10 mol% or less, preferably less than 8 Mol% to adjust. 14. The method according to claim 12, characterized in that g e ke η η draws that carbon dioxide, ethane or ethylene is used as the internal coolant. 1-5. Process according to claim 1, characterized in that the slurry of crystals which is enriched in meta-xylene, from the mother liquor after the Crystallization is removed. 16. The method according to claim 15, characterized in that the slurry of crystals that is enriched in meta-xylene, is removed by filtration or centrifugation. -27-409810 / 1163 73A0R96 17. The method according to claim 15, characterized in that the crystals obtained, the meta-xylene are enriched, can be further concentrated by partial melting or recrystallization. 18. Process for selectively and preferably enriching mixtures of aromatic Cg isomers in meta-xylene by regulation the relative crystallization rates for meta-xylene and para-xylene, so that the para-xylene crystallizes at a slower rate than the rate at which meta-xylene crystallizes, characterized in that one selects a mixture of Cg aromatic isomers in which (a) meta-xylene and para-xylene are essentially eutectic Ratio are present and (b) ortho-xylene in an amount below the eutectic ratio of ortho-xylene and meta-xylene are present, the concentration of para-xylene in the mixture to about 10 col% or less and preferably to less than 8 Μο1- $ ί5 adjusts the mixture cools at a temperature and for a time sufficiently short to form a slurry To obtain crystals that are enriched in meta-xylene, relative to the ratio of meta-xylene and para-xylene, the one obtains in the case of crystallization up to thermodynamic equilibrium, and one the slurry of enriched crystals from the mother liquor, which remains after crystallization is removed. -28- 409810/1 163 7340696 19. The method according to claim 18, characterized in that the slurry is enriched Crystals, which were separated from the mother liquor, are further enriched in meta-xylene by partial melting or recrystallization. 20. The method according to claim 18, characterized in that (a) cooling the mixture of the aromatic Cg-isomers at a temperature of about -65 to -90 ⁰ C for a period of about 5 is performed to 150 min and (b) the concentration of paraxylene in the aromatic mixture Cg isomers is adjusted by adding an amount of diluent to the mixture, the sufficient to lower the concentration of paraxylene therein to about 10 mol% or less, and preferably less than 8 mole percent. 21. The method according to claim 20, characterized in that there is an inner diluent Coolant used for the mix. 22. The method according to claim 20, characterized in that a solvent is used as the diluent with a low melting point, which are easily separated from the Cg isomers by distillation can. 23. The method according to claim 18, characterized in that the concentration of para-xylene in the mixture of aromatic Cg isomers is adjusted by adding this mixture to a number of one or more -29-409810 / 1163 7340696 subjected to successive crystallizations. 24. The method according to claim 18, characterized in that the selected mixture of aromatic Cg isomer is produced by changing the concentration of ortho-xylene contained in the waste filtrate is, and which is about 8 to 20 MoI-Jo ethylbenzene, about 8 to 13 mol-Jb para-xylene, about 15 to 25 mol% Contains ortho-xylene, and the remainder being meta-xylene adjusts about 11 mole percent or less of ortho-xylene. Λ09810 / 1163